To invade target organs, a large number of human and animal viruses, including members of the orthopoxvirus (OPV) genus, breach epithelial surfaces and then use a lympho-hematogenous (LH) route of dissemination through the regional lymph node (LN) and then the blood. OPVs include the cause of smallpox (variola virus; VARV), the smallpox vaccine (vaccinia virus; VACV), and the cause of mousepox (ectromelia virus; ECTV) that the main virology textbooks use as the archetype for LH dissemination. Immunology and virology textbooks also teach us that the primary function of LNs is to serve as sites of lymphocyte priming. However, work by us and others support the emerging concept that LNs also play an essential role in restricting the LH dissemination of pathogens. The major goal of this project is to understand the how this protective response is assembled in vivo. The Specific Aims are: Specific Aim 1. To determine the mechanism whereby CD11c+ CD11b+ inflammatory dendritic cells (iDC) are recruited to the D-LN and contribute to virus control. Specific Aim 2. To identify and characterize the TLR9/MyD88 expressing cells required for the induction of the early anti-ECTV response in the D-LN. Specific Aim 3. To investigate the mechanisms of cytolytic CD4+ T cell induction in the D-LN. With these three Aims we will discover novel mechanisms that permit the control in the D-LN of a virus that spreads lympho- hematogenously from the periphery. This should be applicable to many pathogenic viruses.